1. Field of the Invention
The present invention relates generally to lubricant compositions useful in medium speed diesel engines such as commonly found in railroad locomotives, marine towboats and stationary power applications. These engines frequently have silver bearings which necessitate high TBN lubricant compositions incorporating specialized silver protective agents to protect against wear, extreme pressure and corrosion of silver parts. However, it is well known that zinc-containing wear agents such as the zinc dihydrocarbyldithiophosphates (typically used in passenger cars) cannot be used for this purpose given their incompatibility with silver bearings. Although chlorine-containing silver lubricity agents have been used for silver protection, it is desirable to find alternatives to such chlorinated materials. Thus, the present invention, more particularly, is directed to a lubricating composition having a TBN (total base number) of at least 7 and preferably in the range of from about 10 to about 30, essentially free of zinc-containing wear inhibitors and chlorine-containing silver lubricity agents comprising a major proportion of an oil of lubricating viscosity and a minor amount of (1) a thiadiazole compound having the general formula: ##STR1## where x and y (the same or different) are integers from 1 to 5 and R.sub.1 and R.sub.2 (the same or different) are H or C.sub.1 to C.sub.50 hydrocarbyl; (2) and overbased detergent, preferably at least one selected from the group consisting of overbased alkali and alkaline earth metal sulfonates, phenates and salicylates; and (3) an ashless dispersant.
The invention is further directed to a method for protecting silver parts in an internal combustion engine by lubricating the same with a lubricant composition comprising a major proportion of an oil of lubricating viscosity and a minor amount of the above-described composition. In accordance with the present invention, the thiadiazole-containing lubricant composition provides excellent silver lubricity and obviates the need for chlorine-containing silver lubricity agents.
2. Discussion of the Prior Art
Large numbers of medium speed diesel engines in the United States, as well as other countries, utilize silver-plated bearings. Thus, apart from providing stability against oxidation and protection against the formation of sludge and carbonaceous deposits, crankcase lubricating oils intended for use in medium speed diesel engines must also be formulated with specialized silver protecting agents in order that silver parts in the engine are not attacked either by the additives in the oil or by the dispersed neutralized decomposition products produced during extended engine operation. Such agents, often referred to as silver lubricity agents, protect against extreme pressure, wear and corrosion.
Although it is essential to include a silver lubricity agent in diesel oils intended for use in engines having silver parts, it is well known that such oils must exclude the zinc-containing anti-wear agents mentioned above, such as the zinc dihydrocarbyldithiophosphates, given the known propensity of the latter to damage the silver components of diesel engines. This is explained, for example, in U.S. Pat. No. 4,428,850 (column 1, lines 63-68).
The antagonism between zinc-containing wear inhibitors and the silver parts in diesel engines has been circumvented in the prior art by using alternative silver lubricity compounds, by far the most common of which are the chlorinated hydrocarbons such as shown in Sung, U.S. Pat. No. 4,171,269. However, while the chlorine compounds of the prior art have been shown to be effective in protecting the silver parts of diesel engines, the Occupational Safety and Health Administration in the United States and other public health agencies throughout the world have expressed concern over potential biological effects of chlorinated compounds. Therefore, an incentive exists to develop novel compositions effective in protecting the silver parts of medium speed diesel engines which overcome the problems or potential problems encountered with the zinc-containing and chlorine-containing wear inhibitors.
Unfortunately, overbased alkali and alkaline earth metal detergents, added to provide beneficial cleanliness properties to lubricant formulations are the principal cause of damage to silver parts. Thus, while it is desirable to impart a high degree of alkalinity through the use of overbased detergents, such overbased materials tend to impair the silver protection characteristics of the oil, making it difficult, without resort to the chlorine-containing agents of the prior art, to formulate a diesel lubricant composition which gives the desired level of cleanliness, yet at the same time protects the silver parts of the diesel engine.
A number of patents are thought to be of relevance as background to the compositions and methods described in the present invention, for example, Sung, et al., U.S. Pat. No. 4,256,595, is directed to a diesel crankcase lubricant composition comprising a lubricating oil base and the reaction product of a hydrocarbyl succinic anhydride in which the hydrocarbyl radical has from 12 to 30 carbon atoms, and 5-aminotriazole. Although the background section of the patent states that it is known to employ a thiadiazole as a corrosion inhibitor for diesel crankcase lubricating oil, the use of a thiadiazole compound as the sole silver lubricity agent in lubricating oils requiring such agents for the protection of silver bearings is not disclosed.
Davis, U.S. Pat. No. 4,136,043, is directed to compositions useful for suppression of copper activity and "lead paint" deposition in lubricants. The compositions are produced by preparing a mixture of an oil soluble dispersant (preferably a substantially neutral or acidic carboxylic dispersant) and a dimercaptothiadiazole, preferably 2,5-dimercapto-1,3,4-thiadiazole. As stated at column 4, lines 24-39, the carboxylic dispersants encompass nitrogen bridged dispersants wherein the nitrogen group is derived from aliphatic, aromatic, heterocyclic and carbocyclic amines as well as substituted ureas, thioureas, hydrozines, guanidines, amidines, amides, thioamides, cyanamides and the like. Davis is not relevant to the problem of achieving silver lubricity in lubricating compositions for diesel engines.
Sung et al., U.S. Pat. No. 3,969,235 discloses a lubricating oil composition suitable for use in railway diesel engines in which thiadiazoles can be included as antioxidants, sulfur scavengers and antiwear agents (column 5, lines 16 to 24). Silver lubricity is not addressed in the patent and there is neither disclosed or suggested a chlorine-free lubricant composition in which thiadiazoles can completely replace the chlorine-containing silver lubricity agents of the prior art.
Roberts et al., U.S. Pat. No. 2,703,785 discloses an oil soluble 2,5-dimercapto-1,3,4-thiadiazole in an emulsifiable oil solution also containing an alkali metal salt of an oil soluble sulfonic acid. The patent is not directed to silver lubricity attainment in overbased detergent-containing formulations suitable for use in lubricant oils for diesel engines containing silver bearings.
Blaha U.S. Pat. No. 3,663,561 discloses (column 6, lines 27-36) that 2-mercapto-5-hydrocarbyldithio-1,3,4-thiadiazoles can be added to lubricating compositions to protect sliver metal parts from sulfur corrosion and to provide anti-wear. The patent does not, however, disclose a zinc-free, chlorine-free overbased detergent containing composition compatible with silver engine parts in higher TBN diesel engine applications.
Sabol U.S. Pat. No. 2,749,311 discloses compositions noncorrosive to silver comprising the reaction product of a mercaptan, formic acid and 2,5-di-mercapto-1,3,4-thiadiazole.